In order to reduce the rolling resistance of the tire, a greater part of carbon black compounded in a tread rubber tends to be positively replaced with silica having a low hysteresis characteristic. However, a tread rubber compounded with a great amount of silica becomes higher in the value of electric resistance, and hence there is caused a problem that static electricity generated in a vehicle hardly discharge through the tire toward a road surface.
For the purpose of ensuring the discharge property of the tread rubber is proposed a tire wherein a tread is constructed with at least one tread layer comprising a tread rubber having a low electric conduction and an electrically conductive band arranged in a widthwise middle portion of such a tread rubber and constituting at least a part of a conductive path from a belt to a treading face of the tread. A first proposal is a tire wherein each of the tread rubber and the electrically conductive band constituting the tread layer is made from an annular rubber member of one-piece structure (see JP-A-11-151907), and a second proposal is a tire wherein each of the tread rubber and the electrically conductive band constituting the tread layer is made from a rubber ribbon wound plural times in a circumferential direction of the tread (see JP-A-2000-96402).
However, these proposals have the following problems. As to the first proposal, the tread rubber is naturally formed by winding an elongate rubber extruded through an orifice corresponding to a shape of a tire product on a full periphery of a tire once, so that the thus formed tread rubber has a joint portion at one place of the periphery, which is a cause of deteriorating the tire uniformity. Also, the formation of such a tread rubber needs a large-size extruder but also the orifice set in the extruder once can not be easily exchanged with another new orifice, so that there is a drawback that this proposal can not be applied to an efficient system of mixedly producing tires of various sizes while reducing intermediate stock.
The problem of the second proposal is as follows. The material of the electrically conductive band is different from that of the tread rubber naturally optimized toward the function inherent to the tread, and hence it is preferable to become thin as far as a thickness in the widthwise direction of the tire enough to guarantee the electric conduction is ensured. However, the electrically conductive band of the second proposal has a problem that the band has a structure of winding and laminating a continuous uncured conductive rubber ribbon and the thickness becomes considerably thicker than the naturally required thickness.
Furthermore, at least one of upper face and lower face in the tread layer is an adjacent face adjoining to the other tire constituting layer inward or outward in the radial direction, so that the electrically conductive band is exposed to the adjacent face over the full periphery and is necessary to surely hold the contact with an electrically conductive portion of the adjoining tire constituting layer. In the first and second proposals, however, the electrically conductive band is exposed to the adjacent face as only a narrow-width line, and in this case, a portion not exposing to the adjacent face may be caused due to the scattering in the production. Also, when two of the tread layers having the above structure are laminated, the exposing portions of the electrically conductive bands are not matched with each other in the widthwise direction or it is difficult to ensure the electric conduction among these layers. If it is intended to control the scattering in the production for preventing this drawback, a production equipment of a high precision is required and hence there is caused a problem rendering a significant cost-up.